1. Field of the Invention
This invention relates to a method of extracting selected sweet glycosides from the stevia rebaudiana plant.
2. Description of the Previously Published Art
Japanese Patent 63173531, which issued in 1988 to Nakazato, describes a method of extracting sweet glycosides from the stevia rebaudiana plant. This method includes the following steps. The first step is to extract a liquid solution of sweet glycosides from the stevia rebaudiana plant. Secondly, the liquid solution of sweet glycosides is passed through a non-polar porous resin, such as amberlite XAD-2 and eluting with a water soluble organic solvent, preferably methanol. Thirdly, the eluted solution is concentrated and dried to give a powdery material. This procedure isolates a mixture of sweet glycosides, but it does not isolate pure single compound sweet glycosides and it especially does not isolate pure Rebaudioside A.
Subsequent patents, such as Korean Patent 9007421, have added to or varied the Nakazato process in an attempt to improve the purity and yield. Korean Patent 9007421, which issued in 1990 to Pacific Chemical Company, passes the eluted solution through a column which is packed with positive ion-exchange resin (preferably Diaion SK1B) and negative ion-exchange resin (Amberite IRA 904). Again it appears this is isolating a mixture of sweet glycosides, but it does not isolate pure single compound sweet glycosides and it especially does not isolate pure Rebaudioside A.
U.S. Pat. No. 4,892,938, to Giovanetto discloses a purification process in which the aqueous extracts of the plant are purified by passing these aqueous extracts through a series of ion-exchange resins which are selected to remove various impurities. The sweet glycosides remain in the water and are recovered by evaporation of the water. The advantage is that everything is done in water, while most other processes involve the use of a solvent at some point. The disadvantage is that the final product is quite impure with only about 70% is a mixture of the sweet glycosides. The balance is mainly material more polar than the sweet glycosides which we have identified as a complex mixture of polysaccharides (about 25%), and a small amount of yellow, oily material less polar than the sweet glycosides (about 5%).
This low polarity oil was isolated by chromatography. The flavor of the low polarity oil is very unpleasant. We have found this oil to be present in varying levels from 0.2 to 2.0% in every commercial product we have examined. Since of varying amounts this intensely off-flavored material is contained in the commercial materials it presents problems with quality control and flavor issues. The polysaccharide fraction also appears to contain off-flavor materials, but not as intense in flavor as the low polarity yellow oil.
The sweet glycosides obtained from Giovanetto process are always a mixture. We have determined that the two principle sweet glycosides are Stevioside and Rebaudioside A, and two of the minor sweet glycosides are Dulcoside and Rebaudioside C, although there are many other minor ones. We have isolated the two principle glycosides and we have found that there is a considerably different flavor between them with one being much more desirable than the other. Stevioside has an aftertaste which is undesirable. This aftertaste is present in Stevioside samples of even greater than 99% purity. On the other hand, Rebaudioside A does not possess an aftertaste and has a sweetness flavor comparable to sucrose. Thus it is recognized as having the most desirable sensory properties. In addition to this complexity, various impurities are also present and some of these possess undesirable flavors. The entire matter is further clouded by the extreme difficulty of doing analyzes. The analytical exercise pushes at the envelope of present technology and involves considerable art. Finally, the problem with the methods described above is that the resulting materials contain a mixture of all of the sweet glycosides.